Strip transmission line having phase trimmer means



Dec. 19, 1967 3,359,513

D. J. KELLEY STRIP TRANSMISSION LINE HAVING PHASE TRIMMER MEANS Filed Aug. 51, 1965 FIG. 4 3 Douglas J. Kelley,

INVENTOR.

BY M J. W M

United States Patent 3,359,513 STRIP TRANSMISSION LINE HAVING PHASE TRIMMER MEANS Douglas J. Kelley, Lexington, Mass, assignor, by mesne assignments, to the United States of America as represented by the Secretary of the Army Filed Aug. 31, 1965, Ser. No. 484,139 1 Claim. (Cl. 333-84) ABSTRACT OF THE DISCLOSURE A stripline device defined by a line between grounds and having phase trimmer means for adjusting the electrical length of the stripline. The phase trimmer means is defined by adjustable screws extending through aligned apertures in a ground plate and an adjacent dielectric layer. The screws extend in a plane normal to the stripline and are spaced apart at odd quarter Wave lengths.

This invention relates to improvementsin strip transmission line circuits and more particularly to a stripline phase trimmer as may be used for fine tuning the transmission phase length of strip transmission line circuits. The present invention contemplates the provision of phase trimmer means for fine-tuning by changing the capacity of a section of transmission line within the circuit.

Stripline transmission circuits are presently being used extensively in the ground control components of various missile systems, e.g., in MAR radars and computers.

While stripline phase trimmers heretofore known have generally been satisfactory for most applications, many problems and disadvantages have been encountered with the use thereof. For example, known phase trimmers have been relatively large and expensive, and have required considerable machining. For very small phase shifts, these units are more elaborate than is normally desirable.

Another serious disadvantage in the use of known phase trimmers is the resulting low efficiency wherein below optimum power transfer is effected. A common method, for example, of obtaining small phase shifts economically is to introduce a single screw into the dielectric directly over the transmission line, said screw extending normal to said line. The depth of penetration will determine the phase length of the transmission line and, since it has changed the capacity of the line in the immediate area of the screw, it also will cause the input impedance of the line to change. This change of input impedance is undesirable since a constant impedance circuit is necessary for optimum power transfer. For this latter reason, such a device for phase trimming can only be used in circuits wherein efliciency is not important. The present invention is intended to overcome these and other disadvantages.

Accordingly, the principal object of this invention is to provide an improved phase trimmer adapted for use in fine tuning stripline circuits.

Another object of this invention is to provide a phase trimmer adapted for adjusting the electrical length of stripline circuits whereby fabrication errors due to etching, and machining tolerances can be minimized.

A further object of the invention is to provide a phase trimmer wherein phase path length changes due to dielectric constant variation within the propagation medium can be obviated.

A still further object of the invention is to provide a phase trimmer adapted in use to maintain a constant impedance circuit whereby optimum power transfer is effected.

The foregoing and other objects and features of the invention will become more fully apparent from the following detailed description and from the accompanying drawings, wherein:

FIGURE 1 is a perspective view of a strip transmission line according to the invention;

FIGURE 2 is a sectional view taken on line 22 of FIGURE 1;

FIGURE 3 is a perspective view of another embodiment of the invention; and,

FIGURE 4 is a sectional view taken on line 44 of FIGURE 3.

Referring to the drawing, reference numeral 10 gen erally designates a stripline device including a first cop per ground plate 12, a first dielectric layer 14, a second dielectric layer 18, and a second copper ground plate 20. A copper strip transmission line 16 is imbedded Within the device between the two dielectric layers, said stripline extending along the longitudinal axis of the device.

Ground plate 20 is providedtFIG. 2) with a pair of internally threaded apertures 22, 23. These apertures extend transversely of the plate normal to and directly above strip 16 and are preferably spaced apart longitudinally along the length of the transmission line device a quarter wave length (M4), or odd multiples thereof. Dielectric layer 18 is formed with apertures 26, 27 which are coaxially aligned, respectively, with apertures 22, 23. Apertures 26, 27 are preferably formed of a larger diameter than apertures 22, 23. Adjusting screws 24 are threadedly received within apertures 22, 23 and are adapted for freely extending into coaxially aligned apertures 26, 27.

In operation of the phase trimmer device on a strip transmission line circuit, each screw 24 will introduce the same reactive component for a given depth of penetration. Thus, screws 24 are initially set at the same distance within the apertures and are adjusted the same distance to produce equal reactions. If the distance between the pair of screws 24 is a quarter wave length or odd multiples thereof, the reflected signal due to one reactive component will combine with the reflected signal from the second screw out of phase due to the twice path length relationship of reflected signals. Thus, since the reflected signals are of equal magnitude and 180 out of phase, one will cancel the other resulting in zero reflected signal at the input. Since the input impedance of the circuit has not changed, there still re sults a phase shift through the device because the capacity of the transmission line in the immediate area of the adjusting screws is different at these points than along the remainder of the line. This holds true for small amounts of phase shift only since the excessive penetration required for relatively large phase shifts would result in unequal reflected signals which, in turn, would upset the input impedance. By utilizing this principle of cancellation of reflections, fine-tuning phase trimmers may be easily and economically added to stripline components to overcome fabrication, etching and dielectric tolerances.

A modified form of the invention is shown in FIG- URES 3 and 4. In this embodiment, copper strip transmission line 30 is folded or return-bent into a U-shaped configuration in a plane common with its width (FIG- URE 3). This embodiment further differs from the FIG- URE l embodiment by utilizing a single, larger screw 32 in lieu of the two smaller screws. Thus, the total length of the stripline extending from a plane transverse to the transmission line, through the center axis of the screw on one side, around the bend to the same plane on the other side of the screw equals the required wave length relationship. Hence, a single adjustment is required which introduces two equal reactions, whereas,

Patented Dec. 19, 1967 in utilizing two screws, it is necessary to adjust each screw separately by the same amount for equal reactions. The single screw arrangement simplifies adjusting components for equal phase length. As noted hereinabove, this invention is intended for efiecting small phase adjustments where screw reflections are small and transmission line losses are reasonably low. For example, several thousand of these trimmers have been incorporated into stripline components for a single radar system. In this typical application, a phase trimmer adjustment of :3 has been realized over a 15% bandwidth in L- Band with negligible effect on the input impedance of the component under test.

Accordingly, an improved reactive phase trimmer is provided by the present invention, which trimmer is small, simple, inexpensive, and the same requires minimum machining. The improved phase trimmer is particularly desirable for fine-tuning the transmission phase length of strip transmission line circuits.

It will be apparent also that the embodiments shown are only exemplary and that various changes and modifications may be made in construction and arrangement within the scope of the invention as defined in the appended claim.

I claim:

A stripline device comprising:

a first conductive, longitudinally extending ground plate;

a first dielectric layer in superimposed relation with said first dielectric layer;

a second dielectric layer in superimposed relation with said first dielectric layer;

a second conductive, longitudinally extending ground plate in superimposed relation with said second dielectric layer;

said second plate and said second dielectric layer having coaxially aligned apertures formed therein;

a conductive strip transmission line imbedded within said device therebetween said first and second dielectric layers extending along the longitudinal axis of said device, said strip transmission line being disposed in a plane common with its width and parallel to the adjacent dielectric layers, said line being return-bent at one end thereof so as to form a U-shaped portion;

phase trimmer means adapted for adjusting the electric length of said stripline device, said trimmer means being defined by a single adjusting screw extending through said aligned apertures in a plane normal to the strip transmission line and located equidistantly between legs of said U-shaped portion.

References Cited UNITED STATES PATENTS 2,735,073 2/1956 Grieg 33384 2,820,206 1/1958 Arditi et al 33384 2,896,177 7/1959 Wilson 33384 OTHER REFERENCES Reich, H. 1., et al.: Microwave Theory and Techniques, Van Nostrand Co. Inc., New York, 1953 (pp. 176, 177, 338, 339 relied on).

HERMAN KARL SAALBACH, Primary Examiner.

L. ALLAHUT, Assistant Examiner. 

